1. Technical Field
This invention relates to an ultrasonic contact transducer with multiple elements.
It is particularly applicable to medicine and nondestructive destructive testing of mechanical parts, and particularly parts with a complex shape or irregular surface condition, for example due to grinding.
2. State of Prior Art
In many industrial fields, and particularly in the case of nuclear power stations, testing by an ultrasonic contact transducer plays an important role in material inspection.
This technique consists of moving this ultrasonic transducer directly into contact with a part to be inspected. For each of its positions, the transducer emits ultrasonic pulses and records echoes reflected by the structure and possibly part defects.
However, many geometric aspects make the use of ultrasounds difficult; restricted access (particularly for connections), variable surface conditions and profile variations. Transducers used during these inspections are conventional transducers that cannot optimize the examination.
For example, depending on the areas, sensitivity variations can be observed caused by bad contact between the transducer and the inspected part, or inaccurate positioning may occur due to incorrect orientation of the transducer pressed in contact with the part, or a weld may be only partially covered when the transducer is prevented from moving by the surface configuration.
Therefore, many difficulties are observed during inspections carried out on parts with complex configurations. They illustrate the limits to the performances of conventional ultrasonic contact transducers:
1) Variation of the Thickness of the Coupling Layer
Contact is not optimal when the ultrasonic transducer passes over an area with a nonconforming surface condition or with configuration variations. Thus, the coupling layer located between the surface of the test piece being inspected and the emitting surface of the transducer has a variable thickness. Therefore the delay due to passing through this layer is different for ultrasonic waves emitted from different points on the transducer surface.
Furthermore, complex interference phenomena between the different successively reflected waves occur in this layer. The result is a deterioration of the ultrasonic beam resulting in a loss of sensitivity of the inspection. The capacity of the transducer to detect defects is thus limited.
2) Incorrect Transducer Orientations
The orientation of a transducer pressed on a test piece while being used to make an inspection of a test piece with profile variations, varies during the inspection. Thus, the direction of propagation of the ultrasonic wave in the test piece cannot be controlled since it changes as the transducer is displaced along the profile.
During an inspection carried out in manual mode, the operator cannot make the displacement along a perfectly straight line, which causes another disorientation of the transmitted ultrasonic beam. Information about the position of the defect in the test piece is then lost since the direction of propagation of the beam in this test piece is unknown.
3) Restricted Access
In some cases, the configuration of a part to be inspected makes it impossible to move the transducer along the full length of this part. The area to be inspected can only be partially covered.
We will now examine known solutions for solving these problems.
The ultrasonic beam is controlled by focusing the transmitted beam in the inspected part at a predetermined focusing depth and orientation.
The focusing principle consists of applying delays to the emitting surface such that contributions reach the required focal point in phase.
In the case of monolithic transducers, delays are physically distributed by applying a phase shifting lens formed on the emission surface. Therefore this type of system is fixed, and can only be satisfactory if there are no configuration variations on the part surface.
Dynamic shaping of the ultrasonic beam requires the use of transducers with multiple elements or multi-element transducers. Delays are electronically assigned to each element in the transducer, so that the characteristics of the ultrasonic beam generated by a single element can be modified, and therefore the beam focus can be controlled, and at the same time deformations caused by surfaces with a variable configuration can be compensated.
1) Immersed Multi-element Transducers
An inspection of a part with a variable profile can be carried out using a multi-element transducer immersed in a coupling liquid, for example water. In this case, the transducer is no longer placed in direct contact with the part, but is separated from it by a sufficiently thick layer of water so that interference phenomena between the different ultrasonic waves successively reflected in the coupling layer (a water layer in the example considered) are strongly reduced.
During the inspection of a part with a complex geometry, the ultrasonic beam is focused by calculating the path in water and the material from which this part is made (for example steel) of ultrasonic waves limited by the different elements of the transducer to the focal point, for each position of the transducer.
This solution causes serious difficulties. The adapted delay law cannot be calculated without knowledge of the exact configuration of the part and the position and orientation of the transducer with respect to the part.
Furthermore, this inspection mode cannot always be used in an industrial environment. Local immersion of the part may be difficult, particularly due to restricted access.
2) Multi-element Contact Transducers
Multi-element contact transducers are also used. However, degradations to the transmitted field due to an unsuitable contact are present during the inspection of parts with complex configurations.
Algorithmic techniques have been developed to compensate for this degradation, but they are not very satisfactory because they require that known defects should be present in the part.
One recently developed solution consists of using a multi-element contact transducer with a deformable emitting surface to adapt to the exact surface of the part. In this case contact is optimal, the coupling layer between the emitting surface and the inspected part remains sufficiently thin and uniform to not disturb transmission of the wave.
One particular transducer is obtained from rigid piezoelectric wafers (made of ceramic) embedded in a flexible substrate that is passive to ultrasounds and is described in the following documents:
D. J. Powell, and G. Hayward xe2x80x9cFlexible ultrasonic transducer arrays for Nondestructive evaluation applications PART I: The theoretical modeling approachxe2x80x9d, IEEE transactions on ultrasonics, ferroelectrics, and frequency control, vol. 43, No. 3, May 1996, pages 385 to 392;
D. J. Powell and G. Hayward, xe2x80x9cFlexible ultrasonic transducer arrays for nondestructive evaluation applications PART II: Performance assessment of different array configurationsxe2x80x9d, IEEE transactions on ultrasonic, ferroelectrics, and frequency control, vol. 43, No. 3, May 1996, pages 393-402; and
Internal patent application WO 94/13411, international publication date: Jun. 23, 1994 for xe2x80x9cUltrasonic transducerxe2x80x9d, invented by G. Hayward and D. J. Powell.
However in this case, control of the transmitted ultrasonic beam in order to optimize characterization of defects requires exact knowledge of the geometry of the inspected part and the position and orientation of the transducer with respect to this part.
This invention is designed to improve the performance of the ultrasound inspection of a part with a complex geometry (mechanical part or even part of the human body) in order to better detect, localize and characterize defects in this object.
Improvement of this performance requires control of the ultrasonic beam transmitted in the object, particularly concerning the focusing depth and orientation of this beam.
More precisely, the purpose of this invention is an ultrasonic contact transducer with multiple elements, each element being an ultrasound transmitter and/or receiver, the transducer being designed to be moved with respect to an object to be inspected and with a deformable emitting surface designed to come into contact with the surface of this object, and from which ultrasounds are emitted to the object, control means being provided to generate excitation pulses for emitting elements, this transducer being characterized in that it comprises means of determining the positions of each of the ultrasound emitting elements with respect to the object as the transducer is being moved, processing means being provided to:
generate delay laws, starting from the position thus determined, such that emitting elements can generate a focused ultrasonic beam with characteristics that are controlled with respect to the object, and
apply these delay laws to excitation pulses, the ultrasound receiving elements being designed to supply signals for the formation of images related to the object.
With this invention, it is no longer necessary to know the exact configuration of the object since it is measured by the transducer. The transducer is then capable of operating independently since it adapts to the real configuration of the inspection made by measurement, analysis and compensation of the deformation of the emitting surface of this transducer.
It can thus be considered that this transducer is xe2x80x9cintelligentxe2x80x9d.
According to a first particular embodiment of the transducer according to the invention, multiple elements are formed from a flexible piezoelectric polymer strip and a network of adjacent electrodes obtained by metallic deposition.
According to a second particular embodiment, the multiple elements are rigid piezoelectric elements embedded in a flexible substrate that is passive with respect to ultrasounds.
According to a third particular embodiment, the multiple elements are rigid and assembled to each other mechanically in order to form an articulated structure.
According to a preferred embodiment of the transducer according to the invention, the means of determining the positions of each of the ultrasound emitting elements with respect to the object comprise:
first means designed to determine the positions of each of the emitting elements with respect to a non-deformable part of the transducer by measuring the deformation of the emitting surface, and to provide signals representative of the positions thus determined,
second means designed to determine the position and orientation of this non-deformable part of the transducer with respect to the object and to supply representative signals of the position and orientation thus determined, and
third means designed to supply the positions of each of the ultrasound emitting elements with respect to the object making use of the signals output by these first and second means.
Preferably, the first means comprise:
means of measuring the distance from separate and fixed parts of the non-deformable part of the transducer, from the backing of each element or a subassembly of ultrasound emitting elements, and
auxiliary processing means designed to determine the position of each ultrasound emitting element, making use of the distances determined above.
According to a first particular embodiment of the invention, the distance measurement means comprise:
auxiliary ultrasound emitters fixed to the backings of the elements of the subassembly and designed to emit ultrasounds in sequence,
auxiliary ultrasound receivers fixed to the non-deformable deformable part and designed to detect ultrasounds emitted by the auxiliary emitters, and
means of measuring the distance of each auxiliary emitter from each receiver in a group of auxiliary receivers receiving higher intensity ultrasounds.
According to a second particular embodiment of the invention, the distance measurement means comprise:
a microwave source,
a plurality of microwave antennas rigidly fixed to the non-deformable part, coupled to this source and designed to emit microwaves in sequence, and also to receive microwaves in sequence,
microwave probes fixed to the different backings of elements of the sub-assembly and designed to scatter microwaves emitted by the antennas, these probes being fitted with non-linear devices to modulate the microwaves scattered by the probes, at different frequencies, and
microwave reception means coupled to antennas and designed to measure the distance from each probe to each antenna by measuring the phase of the microwaves scattered by this probe and received by this antenna, these reception means also being designed to distinguish probes from each other by a synchronous detection at the different modulation frequencies.
Preferably, the auxiliary processing means are designed to determine a profile that best passes through the backings of the elements in the subassembly by an interpolation method, and to use this profile to determine the position of the emitting face of each ultrasound emitting element with respect to the non-deformable part of the transducer.
The second means may comprise an articulated mechanical arm fixed to the non-deformable part of the transducer.